Conventionally, in a color printer for printing graphics or image data, color data to be handled is given as RGB values specified by a color mode or a command in the case of a graphics. In the case of image data, the data is given in an RGB dot sequential format or an RGB frame sequential format. Color space for handling color data is not limited to RGB, but there exist YMC color space specific to color printers (depending on ink characteristics), and XYZ color space defined by CIE and the like.
In any case, when printing is performed inside a color printer, inputted data is subject to color reproduction processing corresponding to the color space defined by the color printer (for example, conversion from RGB to YMCK), and then actual print output is performed.
In general, to match colors in the color data handled by a color printer described above with those in color data handled by a color scanner or a color display such as a CRT, a reference color space is used to perform color correction appropriate for respective luminescence (color development) characteristics for each of the color printer and the color scanner or color display.
In this case, the color processing performed in the color printer is in accordance with the reference color space so that, for example, an image displayed on a color display can be outputted to faithfully reproduce it with the color printer.
For example, in order to handle the same color data in devices such as a color scanner, a color display and a color printer, a reference color space, that is, a device-independent color space is defined. After color signals used in the color scanner, the display, and the like are temporarily converted to the device-independent color space, the color space transformation processing corresponding to each device is used to convert the color space to the one specific to each device, so that the color matching can be realized among the devices.
Actually, it is difficult to seek perfect colorimetric matching, since the color reproduction ranges of respective devices are different from one another because of the physical characteristics the devices originally have. In general, color correction is used so that color difference is minimized by a color-difference formula typified by CIE1976 L*a*b* and the like.
However, numerous color-difference formulas for evaluating whether two colors expressed on different media, such as a screen of a color display and recording paper of a color printer, are equal or not have been proposed. The fact is, however, that there is no such thing as an absolute color-difference formula, and in many cases, the color-difference formulas are selected depending on the purpose of use.
Similarly, there also exist some color reproduction methods, which are also selected depending on the purpose of use. When the above-described color matching is considered, the evaluation method naturally differs depending on what color reproduction is desired. Especially, in a color printer, the color reproduction method is an important factor which influences the quality of an outputted printed matter. Generally, the CIE1976 L*a*b* formula or the like, as described above, to perform correction so as to minimize color difference arisen before or after color matching is used. This method is effective when color reproduction of color data read from a color scanner is performed by a color printer. The reason is that the original source is a reflective source (color reproduced on paper) and it is relatively easy to reproduce it with inks of a printing apparatus. Since the physical color development mechanism is essentially the same, color reproduction is easy in comparison with other media though there are problems of difference in ink characteristics and of ink density (tone).
However, in the case of such color as glows on the screen of a display, the physical characteristics themselves are different from those of a reflective source, and there are limitations in seeking for color reproduction by a common color-difference formula. When an image outputted by such a medium is a natural image, color reproduction (rendering intent) generally referred to as preferred matching is often utilized. The preferred matching is used to achieve, away from the viewpoint of whether the reproduced image is isochromatic with the original image, more preferable color reproduction for the most important some colors (for example, human flesh color) in the image.
However, even though such color reproduction is effective in the case of handling data like a natural image, the color reproduction which does not take account of isochromatism may cause trouble in the case of handling data like a computer graphics (CG) image.
If color reproduction processing (rendering intent) can be changed according to data to be processed, the above-described problems can be solved. Thus, a polychrome printing apparatus can be provided which is capable of performing print output with a better image quality by selecting color reproduction processing appropriate for the data to be handled.
However, in the conventional example described above, a problem remains in that it is not possible to clearly define which rendering should be selected and applied even if it is attempted to synthesize the data by applying a different rendering for processing data in which CG data and image data are synthesized.